Not sure how that would work, exactly. MIDI is very different to the sort of sounds a microphone would pick up, which is why you can't just play .mp3s or whatever directly off your Tesla coil, and why it takes some effort to convert music into a usable MIDI format. Whatever the mic was connected to would have to determine what note(s) were being sung, and convert them to MIDI, before sending them to the interrupter. It wouldn't simply be a case of plugging everything in and it working.

There are software package that do convert voice to MIDI, such as Digital Ear real-time. What the output of that software will look like to the interrupter is anyone's guess? I suspect it won't work very well, but I might be wrong - I haven't tried it myself ...